1. Field of the Invention
The invention relates to an amplifier circuit of a capacitor microphone generating an electric signal corresponding to sound.
2. Description of the Related Art
A MEMS microphone is receiving attention as a type of capacitor microphone in recent years. The fundamental structure of this MEMS microphone is a capacitor made of two electrode plates, i.e., a diaphragm and a back plate that are closely faced and disposed, and this structure is formed on a silicon substrate by a MEMS (Micro Electro Mechanical Systems) technique. This MEMS microphone is resistant to a temperature in a normal solder reflow process, and may be soldered on a printed board together with other components, for example. The MEMS microphone may be formed smaller than a general electret capacitor microphone (ECM). For this reason, a device with the MEMS microphone achieves high packaging density and miniaturization.
While the ECM uses an electret element that stores electric charge semipermanently and thus does not need a bias voltage, the MEMS microphone needs a relatively high direct current bias voltage for operation. By applying this bias voltage, a constant electric charge Q is charged in the capacitor forming the MEMS microphone. When the diaphragm vibrates by sound pressure in this state, the capacitance C of the capacitor changes to change a voltage V between the terminals. This change of the voltage V is outputted as a sound signal.
An amplifier circuit of a MEMS microphone is described in Japanese Patent Application No. 2008-153981, as formed using a charge pump circuit as a bias voltage generation source and an operational amplifier as a signal amplification portion. An amplifier circuit of an electret capacitor microphone (ECM) is described in Japanese Patent Application No. 2001-102875.
However, in a case of an amplifier circuit of a capacitor microphone built in a mobile phone, noise by a burst signal may enter the power supply wiring etc, and this noise may be mixed in a signal from the capacitor microphone and inputted to the operational amplifier. Then the noise from the power supply wiring etc is also amplified together with the signal and outputted, causing a problem of degrading the noise characteristic of the amplifier circuit.
Furthermore, in an amplifier circuit of a conventional MEMS microphone, noise of the charge pump circuit provided as a bias voltage generation source may be mixed in a signal from the MEMS microphone and inputted to the operational amplifier. Then similarly, the noise of the charge pump circuit is also amplified together with the signal and outputted, causing a problem of degrading the noise characteristic of the amplifier circuit.